1. Field of the Invention
This invention relates to a wireless communication system, and more particularly to a system for improving the overall battery life of a mobile terminal based on the GSM (Global System for Mobile Communication) Radio Access Technology.
2. Description of the Related Art
Nowadays, due to the technological advance and cost curtailment of a wireless cellular phone, wireless phones are no longer a luxury, but are an absolute necessity. Without wireless cellular phones, billions of people all around the world cannot even visualize performing their daily chores and managing their personal/official life. Thanks to GSM, the radio access technology that has changed the life way, people communicate and remain connected in their otherwise hectic daily schedule. GSM is governed by a set of protocols, which are mutually understood by each User Equipment (hereinafter referred to as “UE”) and the NW (Network/Operator which provides the service) with which it communicates.
GSM is built upon the concept of cellular technology, which divides a given region into a number of “cells”. FIG. 1 indicates the overall GSM system in use. Referring to FIG. 1, we can see the various network elements which together constitute the GSM communication system. The Network Sub System (“NSS”) Domain is responsible for all network control decisions. Base Station Subsystem (“BSS”) Domain is responsible for controlling the connectivity with the end-user terminals. In the following discussion, we will describe the BSS in more detail. Inside the BSS, several “cells” and a Base station Controller (BSC) are included. Each of these cells caters to a group of end-user terminals. All these cells are connected with the BSC. That is, the BSC manages a plurality of cells in one BSS and controls reselections and handovers between cells.
FIG. 2 illustrates the architecture of a cell, in detail. These cells are service areas; each equipped with a Radio Frequency (“RF”) transmitter and an RF receiver. They are commonly constructed within a BTS (Base Transceiver Station). Each of these cells is designed to support a fixed set of UE's and has a region over which it can provide acceptable service.
These cells are designed to overlap their service areas so that there is no “No Service” region remaining, as indicated in FIG. 3, where an ideal cell and a fictitious cell are illustrated. A fictitious cell, used for purpose of mathematical computations, has pure hexagonal shape and has well-defined boundaries, without any overlap. In reality, however, this is not the case and the ideal cells do have overlapped coverage areas.
Cell Reselection Process:
At any point of time, the mobile terminal is catered to by one unique ‘cell BTS’. As the mobile terminal wanders in the region, the quality of service provided by the currently serving cell might deteriorate, for reasons that include the UE moving to the boundary of the current cell or an obstruction in the reception of signal due to an un-even terrain. Under such circumstances, the UE might “camp” on the adjacent cell where it is able to receive better signal and thus can decode downlink information at a more acceptable success rate. This process is called “Cell Reselection”. At any point of time, the UE can have a maximum of 32 neighboring cells, as dictated by the protocol specifications.
This cell reselection process is indicated in FIG. 4. Referring to FIG. 4, as the cell draws close to the boundary of the “Rockville cell”, the signal strength and/or signal to noise ratio will decrease and might result in some deterioration of the received message. That is when the mobile terminal reselects to the “Bethesda cell”. The “Rockville cell”, as part of its system information parameters, sends the list of all the neighbor cells which any UE present in its area must monitor.
To this end, the UE, during its time in the current serving cell, must monitor and determine which the neighboring cells are. Furthermore, it also needs to know which one, among all the neighboring cells, has the best ‘received signal level’ strength. This process is called “Neighbor Cell Monitoring”.
Standby Time of Battery and RF Reception in Idle Mode:
A large amount of battery power is used in the process of monitoring the neighboring cells and keeping the database of neighboring cells updated, as stated above. In fact, the process of RF reception to monitor the neighbor cells, in idle mode, is probably the most severe contributing cause for high average current drain. This directly impacts the average standby time of the UE, as evident from Equation (1) below:
                              Standby          ⁢                                          ⁢          Time                =                              Battery            ⁢                                                  ⁢            Capacity            ⁢                                                  ⁢            in            ⁢                                                  ⁢            miliampere            ⁢                                                  ⁢            hours                                Average            ⁢                                                  ⁢            Current            ⁢                                                  ⁢            in            ⁢                                                  ⁢            milliampere                                              (        1        )            
Referring to Formula (1), even a minor reduction from 0.5 to 1.0 milliampere can produce an improvement of standby life by ˜20 to ˜40 hours, using an 800 mA-hour battery.
The idle mode RF reception can be expressed in Equation (2) below:RF reception in idle mode=(RF reception of serving cell paging channel)+(RF reception of neighbor cell broadcast channels)  (2)Discontinuous Reception, DRX:
In conventional systems, an attempt has been made to mitigate the problem of battery drain by mandating an optimized approach called “DRX”, whereby the UE is no longer required to monitor all the continuous occurrences of paging channel in the serving cell. It introduced the concept of a paging group, whereby every UE is part of a particular group. The UE is able to autonomously decide its own Paging Group based on some well-known parameters. With this segmentation in place, the NW is able to send Paging Messages for an entire group at specific intervals. The interval period is informed to all UE's in a given cell as part of the cell information. Now the UE no longer needs to monitor all the Paging Channels; but only when it knows that its own paging group is scheduled. Thus, for the remaining period, the UE can shut down the RF reception unit.
Neighbor Cell Scanning with DRX:
To ensure that the performance of the UE is not degraded and “cell reselection” process is not adversely affected, the UE, at each wake-up, also schedules the monitoring of the neighboring cells and updates its neighboring cell information database. Due to processor speed and timing constraints, the UE schedules the monitoring of 6 to 8 neighbor cells every time it wakes up. Typically 7 cells are monitored, as suggested by the protocol standards. After all the neighboring cells have been monitored, up to a maximum of 32, the UE maintains a sorted list of the six best cells. The sorting is done in terms of received signal level only. This updated list enables the UE to make quick shifts to a neighbor cell whenever the service quality of the current cell falls below an acceptable limit.
Mechanism to Reduce Neighbor Cell Scanning in Idle Mode:
Certain aspects of conventional systems have attempted to cut down the scanning of neighbor cells, thereby RF reception, based on received signal level and received signal quality of serving cell; their fluctuations and the rate of fluctuations; vehicular speed estimation of the UE by making use of sensors and finally decides whether the UE can avoid the scanning of the neighbor cells. Conventional schemes base their decision to scan neighbor cells based on the speed of the UE and what the UE perceives at its antenna. An UE with lower speeds need less scanning of neighbor cells. UE with high fluctuation in the raw signal parameters will resort back to the normal behavior and will not attempt to reduce the neighbor cell scanning.
U.S. Pat. No. 6,526,286 monitors variations of raw signal parameters (e.g., RSSI, SNR); estimating vehicular speed to decide if the frequency of neighbor cell scanning could be reduced.
U.S. Pat. No. 6,292,660, suggests an adaptive site scanning based on the fading rate of the received signal level. The frequency of site scanning has been suggested to be proportional to the fading rate. An increased fading rate increases the rate of site scanning. Yet again, this conventional system attempts to conserve battery by monitoring the fluctuations of raw signal parameters and vehicular speed estimation.
U.S. Patent Publication No. 20050096053, outlines a method whereby the “Ec/Io” or the SNR of the serving cell is being monitored and checked if it is above a certain pre-defined range, its variations have been monitored to finally decide if the frequency of neighbor cell scanning could be reduced.
As with the previous ones, this conventional system suffers from similar drawbacks thereby providing sub-optimal results. As an example, all the above systems would stop the battery conservation procedure if the signal level or quality fluctuates heavily or if the SNR falls below the threshold range.
Reuse and Co-Channel Interference:
As an introduction, it is also mentioned here that due to a limited number of ARFCNs (Absolute Radio Frequency Channel Numbering) granted to an NW operator, the NW operator uses a set of unique frequencies in a group of cells after which the same set of frequencies is re-used at a certain distance. This concept is shown in FIG. 5 with a cluster size of seven. FIG. 5 illustrates a network configuration where a set of seven frequencies, numbered 1 through 7, are re-used to form a meshed network. This set of cells where a unique set of ARFCNs is used is called “Cluster Size—K” and the distance at which the re-use occurs is called “Reuse Distance—D”. This re-use distance is kept such that the signal interference caused by like-frequencies is below tolerant limits.
Given the cell radius (R), and the cluster size (K), the re-use distance can be ideally calculated using the following Equation (3):D=√{square root over (3sKsR)}  (3)
FIG. 6 illustrates this concept. Referring to FIG. 6, co-channel interference is the scenario which occurs when signals corresponding to one of the neighboring cells of the UE interferes with signals of the same frequency that are being re-used in the adjacent cluster as a different cell. Standard equations known in the art are also used to determine the cluster size.
Therefore, there is a need to provide an optimized cell search method for increasing the overall battery life of a mobile terminal within the parameters of the GSM radio access technology.